1. Field of the Invention
The present invention relates to an anti-foam additive for a cementitious composition, to an aqueous solution incorporating said additive, and also to the use thereof in mortars or concretes.
2. Description of the Related Art
The formulation of high-performance concretes often requires plasticizers and superplasticizers either to increase the fluidity of the base cementitious composition at constant water input enabling the improvement of its workability, or for reducing the water content of the cementitious composition at equal consistency with a view to improving the mechanical strength and the durability.
is Among the plasticizers and superplasticizers, use is very often made of molecules chosen from polynaphthalene sulfonates, polymelamine sulfonates, lignosulfates (LS) and polycarboxylates, in particular polycarboxylates with polyethylene oxide side chains (PC-PEO) generally introduced in the form of an aqueous solution into the cementitious composition during the mixing and blending of concretes and mortars. The plasticizer and superplasticizer molecules have surface properties which give rise to air entrainment in the cementitious composition. This occluded air induces a modification of the rheology of the cementitious composition and a not inconsiderable drop in the mechanical strength both in the short term and in the long term of the mortar or concrete thus produced.
This phenomenon is observed more extensively with molecules of the LS and PC-PEO families.
In order to limit inclusion of air in the cementitious composition, anti-foams are generally introduced directly into the solutions of superplasticizers of LS or PC-PEO type so as to prevent the formation of air bubbles or to promote their destruction if they form.
Two constraints are linked to these anti-foam molecules. The first relates to the need to disperse the anti-foam in the aqueous solution of superplasticizer in order to sell a single product that provides the two functions (fluidification of the cementitious composition without excessive air entrainment), whilst the second constraint relates to the actual effectiveness of the anti-foam molecule with respect to the prevention or destruction of occluded air.
Anti-foam agents or defoaming agents are identical molecules, which only differ by their method of introduction into the medium. Specifically, anti-foam agents are introduced before the appearance of the foam in order to prevent the formation of stable gas/liquid interfaces, whilst defoaming agents are introduced after formation of the foam in order to eliminate it.
The present invention relates rather to the formation of anti-foam agents, that is to say additives intended to be introduced at the same time as the superplasticizer solution, into the cementitious composition, based on cement and water, at the time it is blended.
Anti-foam agents are, in general, hydrophobic formulations that are in the form of small-sized droplets that are insoluble beyond a limit in concentration in the liquid constituting the continuous phase, here the aqueous phase. The anti-foam agent is therefore by nature insoluble in the aqueous medium in which it forms an emulsion.
In order to be effective, the anti-foam agent must then migrate to the air/liquid film interface. During this migration, the anti-foam/liquid film and air/liquid film interfaces are replaced by an anti-foam/air interface. This transition requires a hydrophobic anti-foam agent since a hydrophilic formulation would remain within the aqueous phase.
The use of anti-foam agents for cementitious compositions such as cement grouts, mortars or concretes therefore requires dispersing the anti-foam agent in the aqueous superplasticizer solution. Since the anti-foam is by nature a hydrophobic molecule, insoluble in the aqueous medium, it is necessary to produce an emulsion of anti-foam in the aqueous superplasticizer solution.
In order to be marketable, this emulsion of anti-foam in the superplasticizer must be stable over the entire storage life (i.e. one year approximately) and over the range of temperatures to which the superplasticizers are commonly exposed (5-40° C. knowing that in general the emulsion is even less stable when the temperature is higher).
Furthermore, the anti-foam must also be effective in the cementitious medium over the entire processing time (i.e. from 15 minutes to several hours).
In addition, a loss of activity may be caused by a degradation of the anti-foam in the extremely basic medium of the cementitious compositions. The anti-foam molecules must be capable of withstanding this.
Moreover, “the depletion” of the anti-foam over time (resulting in a loss of effectiveness over time) is a well-known phenomenon which must not occur over the period of processing the cementitious compositions. This phenomenon often results from the reduction in size of the anti-foam droplets over time due to the shearing to which they are subjected.
In addition, the anti-foam must be able to be used with the conventional metering equipment. Certain anti-foam agents, such as tributyl phosphate, cause, for example, a degradation of certain pump bodies.